A conventional component-embedded printed wiring board (PWB) is described hereinafter. FIG. 24 shows a structure of the conventional PWB which includes electronic components built therein. As shown in FIG. 24, the conventional PWB comprises plate 1 made of metallic material, and substrates 2a-2e formed of thermoplastic resin and layered over metallic plate 1.
Holes 4 are opened through substrates 2c and 2d for embedding electronic component 3. Patterns 5 are provided on substrates 2a-2e, and via-holes 7 opened through substrates 2a-2e are filled with conductive paste 6. Electrodes 8 placed at both sides of component 3 are conductive to paste 6.
Conductive paste 6 is made by mixing tin grains with silver grains. Between component 3 and holes 4, clearance of 20 μm is provided surrounding component 3 for accurately positioning electrodes 8 with respect to via-holes 7 filled with paste 6. Thus it can be said that the outside dimension of component 3 is approx. equal to 20 μm.
The foregoing conventional PWB undergoes pressing and heating at 1-10 Mpa, 250-350° C. and in 10-20 minutes before completed. In other words, this pressing and heating process melts the tin to be unified with silver, and connects the tin to electrodes 8 of component 3 for fixing component 3 electrically and mechanically. The conventional component-embedded PWB is disclosed in, e.g. Japanese Patent Unexamined Publication No. 2003-86949.
The conventional PWB, however, has the following problem if components 3 are densely mounted. For instance, as shown in FIG. 25, electronic components 3a-3e are mounted at a narrow pitch to substrate 2c, and assume that an interval between the components adjacent to each other is 100 μm. FIG. 26A shows sectional views of substrates 2c and 2d, and FIG. 26B shows an enlarged view of the vicinity of components embedded. As shown in FIGS. 26A and 26B, width W1 of frame 10a placed between electronic components 3a and 3b is found by equation 1.W1=W0−W2×2  (1)
where W2 is, e.g. a distance between component 3a and substrate 2c surrounding component 3a. In this case, since W0=100 μm and W2=20 μm, W1 becomes 60 μm, i.e. the width of frame 10a is 60 μm.
Thickness T1 of substrate 2c is 75 μm, so that width W2 of frame 10a becomes smaller than thickness T1 of substrate 2c, and it becomes physically difficult to manufacture this conventional PWB.
To overcome this problem, holes 13 surrounding components 3a-3e mounted at narrow pitches can be provided as shown in FIG. 27 (plan view) and FIG. 28 (sectional view). In this case, however, space 14a between components 3a and 3b cannot be filled sufficiently with resin 15, so that air 16 sometimes remains. If substrate 2c in this status undergoes soldering in a reflow-oven, the reflow-temperature expands air 16 for applying heavy load between components 3a and 3b. The load has the possibility of damaging the connection of component 3, to be more specific, the conduction of paste 6 is cut, or component 3 sealed with resin produces cracks into which water leaks for rusting electrodes 8 and causing defective insulation.